Actinium

Actinium is a radioactive metallic element with atomic number 89. Its scarcity and intense radioactivity make it useful chiefly as a source of short-lived isotopes, including in targeted cancer therapy.

Actinide: An element in the f-block series from actinium through lawrencium, characterized by filling 5f orbitals. Actinium gives this element series its name and marks its conventional beginning.
Alpha decay: Radioactive decay in which a nucleus emits a helium-4 nucleus, reducing its proton number by two. Actinium-225 initiates a chain of alpha emissions that supplies the therapy’s cytotoxic radiation.
Uranium ore: Rock containing uranium minerals in concentrations sufficient for extraction or study. Natural actinium occurs only in trace quantities within uranium-bearing material.
Actinium-225–PSMA therapy: An experimental targeted alpha therapy pairing actinium-225 with molecules that bind prostate-specific membrane antigen. It is a prominent clinical application of actinium-225 for advanced prostate cancer.
Atomic number: The number of protons in an atom’s nucleus, which identifies its chemical element. Actinium’s 89 protons distinguish it from every other element.
Decay chain: A sequence of radioactive transformations through which unstable nuclides become progressively more stable. Several alpha-emitting daughter nuclides contribute to actinium-225’s therapeutic effect.
Uranium-235 decay series: The sequence of radioactive transformations beginning with uranium-235 and ending in stable lead-207. Actinium-227 is a short-lived intermediate in this natural decay series.
Prostate-specific membrane antigen: A cell-surface protein highly expressed by most prostate cancers and targeted by imaging and therapeutic agents. PSMA-binding molecules can carry actinium-225 to prostate cancer cells.
Radioactive decay: The spontaneous transformation of an unstable atomic nucleus, often accompanied by emitted particles or radiation. Actinium’s radioactivity arises from unstable nuclei transforming into other nuclides.
Half-life: The time required for half the radioactive nuclei in a sample to decay. Actinium-225’s roughly ten-day half-life sets the timescale for treatment and handling.

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